1. Field of the Invention
This invention relates to a reciprocating compressor, such as a fixed capacity swash plate compressor, a variable capacity swash plate compressor, a wobble plate compressor, and an in-line compressor (crank compressor).
2. Description of the Prior Art
FIG. 1 is a partially sectional view showing a piston and a cylinder bore of a conventional variable capacity swash plate compressor.
The piston 107 is slidably received in the cylinder bore 106 formed through a cylinder block 101. When torque is transmitted from a drive source, not shown, the piston 107 reciprocates within the cylinder bore 106. As a result, the volume of a compression chamber within the cylinder bore 106 changes, whereby suction, compression and delivery of refrigerant gas are carried out sequentially.
The cylinder block 101 and the piston 107 are both formed of an aluminum-based material. The piston 107 has its peripheral surface covered with a ferrous thermal sprayed coating 170 for preventing abrasion and seizure of the piston 107.
However, the aluminum-based material forming the piston 107 has a coefficient of linear expansion (Al=2.0.times.10.sup.-6 mm/.degree. C.) different from a coefficient of linear expansion (Fe=1.1.times.10.sup.-6 mm/.degree. C.) of a ferrous material forming the thermal sprayed coating. Therefore, when the temperature within the cylinder bore 106 rises (during operation of the compressor), an inner peripheral surface of the cylinder bore 106 expands at a greater rate than the peripheral surface of the piston 107, and hence a clearance between the peripheral surface of the piston 107 and the inner peripheral surface of the cylinder bore 106 becomes larger. As a result, blow-by gas flowing from the compression chamber is increased in flow rate, which degrades compression efficiency and hence performance of the compressor.
FIG. 5 shows that the clearance between the peripheral surface of the piston 107 and the inner peripheral surface of the cylinder bore 106 is increased according to a rise in the temperature within the cylinder bore 106. A clearance A (see FIG. 1) at an ordinary temperature (10.degree. C.) is 0.010 .mu.m, while a clearance B (see FIG. 1) at a higher temperature (150.degree. C.) is 0.050 .mu.m. This means that the clearance is increased by 0.040 .mu.m when the temperature rises by 140.degree. C. from the ordinary temperature. The increase of the clearance results in an excessive increase in the flow rate of blow-by gas, which degrades compression efficiency of the compressor.